Trace,
Our Us Too International prostate cancer education and support group had its meeting last night, and by good luck I got some timely information. We had a panel of two - a DaVinci robotic surgeon and a radiation oncologist. Both were somewhat skeptical and concerned about CyberKnife, though they struck me as reasonably objective and not trying to shove it out of the prostate cancer treatment marketplace. I'm inserting some comments in green.
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Because CyberKnife is another form of IMRT, but with far more potential beaming aiming points (think aiming from any point in a sphere in contrast to aiming from any point in a circle), my hunch is that its long term results will be close to those from advanced IMRT.

The doctors thought the same, more or less: similar effectiveness to IMRT. Even though the delivered dose is about half as much, there is a formula that radiation oncologists use that indicates the impact on cancer will be about the same because the doses are only a day apart and a much higher amount of radiation is delivered in each of the five doses. However, they were quite concerned that there will not be any long term results of cure rates for years - CyberKnife for localized prostate cancer treatment is just too new.
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Here's something that puzzles me: external beam radiation, which includes CyberKnife, is divided up into sessions over time to give healthy cells a chance to recover, with the time periods being too short for recovery of cancer cells, which are more sensitive to radiation. Well, CyberKnife sharply reduces the number of sessions, with more radiation delivered in each, at least that's my impression. That isn't consistent with my notion of how the healthy cells tolerate the radiation (in other words, the healthy cells get a pretty hefty dose each session), but the early results for side effects look good, so apparently toleration is good. Maybe more aiming points is key. ...

This is where I'm now really concerned about CyberKnife - the long term side effects. Radiation has two kinds of side effects, one, referred to as "acute," develops fairly quickly and then goes away. Another kind takes a while to develop, often several years, but then may stay, including such complications as scaring, fibrosis, and strictures. The studies that have been done with CyberKnife for prostate cancer are short, with the longest having about three years of followup according to our speaker (and what I looked at on [url]www.pubmed.gov[/url]). That's long enough to assess success with short-term side effects, and CyberKnife is doing well on those. However, the doctor said that heavy dosing during individual radiation sessions - the pattern for CyberKnife use, does not affect short-term side effects much, but such heavy dosing is known, based on work with other forms of radiation, to affect long-term side effects. The doctor confirmed that there just has not been long enough followup to know what CyberKnife will do for long-term side effects. He, as well as the robotic surgeon on the panel, were quite concerned, and the long-term risk sure looked like a huge and risky unknown to me also. The doctors felt that CyberKnife was best used in a clinical trial setting, where results would contribute to knowledge. They discouraged other use though, feeling that CyberKnife for local prostate cancer treatment was "experimental at best."

(Coincidentally at the meeting, one of my buddies described a long-term side effect from brachytherapy several years ago that he had recently developed.)

Here's some more technical information about the way CyberKnife is used. While it could be used like current equipment that spreads small doses over many weeks, it is being used and marketed as a quick kind of external beam radiation therapy, the kind you can complete in just five sessions during a single week. There is a name for the dosing and timing used in this approach: Stereotactic Body Radiation Therapy; instead of a standard external beam dose of about 1.8 Gray (radiation units) delivered in 43 sessions for a total dose of 77.4 Gray, spread out over about eight to nine weeks with weekends off, Stereotactic Body Radiation Therapy delivers five doses of 7.25 Gray each within one week for a total dose of about 36.25 Gray. Other types of radiation equipment have also been used to deliver this kind of therapy.

While the new comments are negative, it is always wise to hear the other side before drawing conclusions. I hope we will hear from someone who can defend CyberKnife. A couple of things are clearly in its favor: it is both noninvasive and takes only one week to deliver.